1. Field of the Invention
The present invention relates to a screw compressor, especially relates to a screw compressor that effectively reduces power losses during unload operation.
2. Description of the Related Art
For a compressor equipped with a suction adjustment valve on the suction side of a compressor main body and a gas release valve on the discharge side thereof, the discharge capacity is adjusted by load/unload control operation in which when the pressure on the discharge side becomes high, the suction adjustment valve is closed and the gas release valve is opened, and when the pressure on the discharge side becomes low, the suction adjustment valve is opened and the gas release valve is closed.
However, the above compressor cannot completely close a suction channel with the suction adjustment valve, even in case of unload operation in which compressed gas is not required on the consuming side. When the suction channel is closed, the compression ratio becomes infinite, and the temperature of discharged gas increases. As a result, the operation cannot be continued, and, in the worst case, the compressor main body is damaged. Thus, even in the unload operation, the suction adjustment valve cannot be completely closed, and 20 to 30% of the power that is consumed in load operation is unfortunately wasted.
Thus, various methods for operating compressor have been proposed to solve problems such as the prevention of excessive increase of gas temperature on the discharge side, and the reduction of power losses in unload operation.
For example, Japanese Patent No. 3837278 discloses a method for operating compressor, in which when rotation speed of a motor for driving a compressor main body is higher than a set value that is predetermined, the rotation speed of the motor is controlled by opening a suction adjustment valve on the suction side so that the rotation speed is reduced based on detected values of a pressure sensor on the discharge side. When the rotation speed reaches the set value, the control of the rotation speed is stopped and switched to the other control in which the suction adjustment valve is opened or closed in response to the pressure variation on the discharge side so that the pressure variation on the discharge side is suppressed.
According to this method for operating compressor, the prevention of excessive increase of temperature on the discharge side and reduction of power losses can be realized.
However, there is room for improvement in the reduction of power losses under a state in which the supply of compressed gas is hardly required.